1. Field of Invention
This invention relates to light-sensitive photothermographic emulsion layers containing a thermoreversible organogel based binder. This invention also relates to processes for the application of photothermographic coatings to a substrate and more particularly, it relates to a process for the application of at least one layer of a molten, thermoreversible organogel to a substrate.
2. Background to the Art
Photothermographic imaging materials (i.e., heat-developable photographic materials) that are classified as "dry silver" compositions or emulsions comprise: (1) a photosensitive material that generates atomic silver when irradiated, (2) a light-insensitive, reducible silver source, and (3) a reducing agent for the reducible silver source. The light-sensitive material is generally photographic silver halide which must be in catalytic proximity to the light-insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that atomic silver (Ag.degree.) is a catalyst for the reduction of silver ions, and a progenitor of the light-sensitive photographic silver halide may be placed into catalytic proximity with the light-insensitive, reducible silver source in a number of different fashions, such as partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049), and other methods that intimately associate the light-sensitive photographic silver halide and the light-insensitive, reducible silver source.
The light-insensitive, reducible silver source is a material that contains silver ions. The preferred light-insensitive reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as light-insensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the light-sensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e. the latent image. This produces a black and white image.
As the visible image is produced entirely by silver atoms (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
One conventional way of attempting to increase the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. Color images can be formed by incorporation of leuco dyes into the emulsion. Leuco dyes are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure 29963.
Simultaneous multilayer coating of aqueous gelatin/silver halide emulsions ("photographic emulsions") has been used extensively in the manufacture of photographic films. Photographic emulsions contain aqueous gelatin solutions containing dispersed silver halide grains. In color photographic emulsions, there are present color couplers which are spectrally matched to the sensitization of the silver halide grains. These color couplers are, in turn, contained in dispersed droplets of a water insoluble oil. The individual color coupler molecules have attached oleophilic "hallasting groups", such as tertiary amyl groups, which ensure that the coupler molecule remains dissolved in the oil droplet rather than dissolving into the aqueous phase from which it can undergo interlayer diffusion.
It is essential that the color couplers remain confined within their assigned layers in close association with their correspondingly sensitized silver halide grains. Were the coupler to migrate into a different color layer and react with the wrong silver halide grain, false color renderings would occur (commonly known as "cross-talk").
Simultaneous multilayer coating has the primary advantage of reducing the number of coating steps needed to prepare multi-layered articles. The process for simultaneously applying aqueous gelatin emulsions to form a multilayer film generally involves extruding gelatin emulsions at a temperature above their gel point and then simultaneously coating the extruded gelatin solutions onto a moving web using a coating apparatus (e.g., a slide-hopper). Upon contact with the web, the gelatin-based layers are rapidly cooled below their gel temperature, thereby gelling the individual layers (wherein a rapid qualitative change from liquid to solid properties occurs) and minimizing drying related defects, especially mottle. Subsequently, the coated gelled film is dried to remove excess water.
In conventional polymer solution coating operations (regardless of whether the coating solvent is water or an organic solvent) the newly applied coating undergoes a progressive change from liquid to solid as the solvent evaporates. The evaporation process produces a viscosity change as the polymer concentration increases. Ideally, this viscosity increase would be uniform throughout the coated layer and that layer would be uniformly converted from a liquid layer to a solid film attached to the substrate. Unfortunately, depending upon the evaporation characteristics of the solvent, there is a tendency for the coating to lose solvent more rapidly at or near the surface. This phenomenon and attendant shrinkage applies non-uniform physical stresses to the drying layer. This can be used to produce novel and desirable effects such as a "crinkle finish" on painted surface. More often, the effect is undesirable (e.g., "orange peel" in the automotive paint field). In polymeric coatings applied to a moving substrate, this is manifested as striations or mottle which is analogous to the aforementioned "orange peel". When the coating solvent is a volatile organic solvent of high vapor pressure, the tendency is amplified.
When the coating is rapidly converted to a solid, prior to solvent evaporation, the coating resists stress deformation and the defects caused thereby. The field of gelatin/silver halide photography has taken advantage of this by utilizing the ability of molten gelatin emulsions to "chill set" to a solid gel from which the solvent (water) diffuses while maintaining the original smooth topography of the coating.
Gelation allows greater flexibility in the coating and drying process, allowing for drying conditions with higher air flow rates and conditions that in a non-gelling coating would be very difficult. "Air turnarounds," commonly used in photographic drying operations to avoid roller contact with the gel layer, would be impossible in a non-gelling coating. The layer would be blown off the substrate. The advantages of gel coating become more apparent as the wet thickness of single coatings or the number of simultaneously coated layers increases.
Until now, there has been no disclosure of simultaneously applying organic solvent-based coatings, which can be cooled to organogels, to suitable substrates.
U.S. Pat. No. 4,966,792 describes stacked aqueous gel-forming solutions (e.g., acrylamides) of varying concentration gradients for use in electrophoresis. There is no disclosure of using non-aqueous-based gels.
U.S. Pat. No. 4,525,392 discloses a method for simultaneously applying multiple layers of gelatin solutions to a web. A slide-hopper type coating apparatus is used to coat the solutions. Interlayer mixing is controlled by adjusting the relative flow viscosities of the aqueous gelatin layers flowing on the slide surface.
U.S. Pat. No. 4,384,015 and U.S. Statutory Invention Registration H1003 disclose processes for the simultaneous coating of multiple aqueous gelatin-based layers for photographic applications.
U.S. Pat. No. 3,920,862 discloses multilayer coating of aqueous gelatin solutions incorporating a stripe of recording material.
U.S. Pat. No. 4,791,004 discloses a method for forming multi-layered coated articles by increasing the viscosity of a coated solution followed by a lamination step.
U.S. Pat. No. 4,684,551 discloses an apparatus useful for coating thixotropic polyvinyl fluoride as a plastisol in a latent solvent (i.e., a liquid dispersing agent that becomes a true solvent upon heating). No mention of multiple coatings is made.
U.S. Pat. Nos. 2,647,296 and 2,647,488 disclose a method for coating textile fabric with a polymeric plastisol composition.
U.S. Pat. Nos. 2,419,008, 2,419,010, 2,510,783, 2,599,300, 2,953,818, and 3,139,470 disclose processes for the manufacture of films from orientable polyvinyl fluoride. Those processes involve extrusion of polyvinylidene fluoride dissolved in a solvent. A solvent is mixed with polyvinylidene fluoride and heated until the polyvinyl fluoride particles coalesce. The uniform mixture is extruded and upon rapid cooling forms a self-supporting film which can be further dried.
U.S. Pat. No. 4,281,060 discloses the use of polyisocyanate hardeners to improve multilayer coatability of silver halide-containing photothermographic layers having poly(vinyl butyral) binders.
European Patent Application No. 388,818 discloses a dual slot extrusion coating die for use with non-aqueous coating compositions.
U.S. Pat. No. 3,985,565 (to Gabrielsen et al.) discloses photothermographic elements containing binders with poly(vinyl butyral) dissolved in organic solvents such as acetone/toluene blends, but with the subsequent addition of methanol no gelation will actually occur as demonstrated by various example later herein.
U.S. Pat. No. 4,022,617 (to McGuckin) also discloses a photothermographic element wherein a binder is employed containing poly(vinyl butyral) dissolved in an acetone/toluene solvent blend, but with the subsequent addition of ethanol no gelation will actually occur as demonstrated by example later herein.
What would be desirable in the industry is light-sensitive photothermographic emulsions layers containing an organogel based binder. What would also be desirable in the industry is a process for the preparation of light-sensitive photothermographic emulsion layers containing a thermoreversible organogel based binder.